Reading the result
| Evaporation rate | What to expect |
|---|---|
| Under 0.1 lb/ft²/h (0.5 kg/m²/h) | Plastic shrinkage cracks not expected |
| 0.1–0.2 lb/ft²/h (0.5–1.0 kg/m²/h) | Cracks may occur — precautions recommended |
| Over 0.2 lb/ft²/h (1.0 kg/m²/h) | Cracks expected — precautions needed |
The equation
The calculator uses Uno’s 1998 equation, which reproduces the ACI 305R nomograph:
US units are converted to metric, run through the equation and converted back (1 kg/m²/h = 0.205 lb/ft²/h). Uno also published an English-unit form, E = (Tc^2.5 − r × Ta^2.5)(1 + 0.4V) × 10⁻⁶ in lb/ft²/h with °F and mph; the two agree to within about 10%.
Worked example
85 °F air and concrete, 40% humidity, 10 mph wind: about 0.19 lb/ft²/h (0.93 kg/m²/h) — the top of the “take precautions” band, close to where cracks are expected.
What helps
NRMCA’s list of precautions for plastic shrinkage cracking:
- Put up temporary windbreaks and, if you can, sunshades.
- Use fog sprays to put a fine mist into the air above the concrete, upwind.
- Dampen the subgrade before placing — but leave no standing water.
- Dampen the forms and reinforcement.
- Have enough people, tools and materials on hand to place and finish promptly.
- Cover the concrete with wet burlap or polyethylene sheeting between finishing steps.
- Start curing as soon as possible — membrane curing compound or wet burlap.
- Consider synthetic fibers (ASTM C1116) in the mix.
Plan the pour time
Early morning is usually cooler, calmer and more humid than midafternoon. Running the calculator for both times the day before can be the cheapest precaution of all.
Questions people ask
What evaporation rate causes plastic shrinkage cracking?
Below 0.1 lb/ft²/h (0.5 kg/m²/h) cracks aren’t expected. Between 0.1 and 0.2 lb/ft²/h they may occur and precautions are recommended. Above 0.2 lb/ft²/h (1.0 kg/m²/h) they’re expected unless you protect the surface. The bands trace back to work by Carl Menzel at PCA and Delmar Bloem at NRMCA, referenced in ACI 305.
Why does the concrete’s own temperature matter?
Evaporation depends on the difference between the vapor pressure at the concrete surface and in the air. Warm concrete on a cooler, dry day evaporates fast: 80 °F concrete in 70 °F air at 50% humidity and a 10 mph wind loses water about 1.8 times as fast as when both are 70 °F.
When should I check?
Before the pour and again while you’re placing and finishing — wind and humidity change through the day. Midday sun and a rising wind can move a slab from one band to the next within an hour.
Is this exact?
No — it’s an estimate of the same kind as the ACI 305R chart, which it reproduces. Shade, sun on the surface and the mix itself all matter. Treat the bands as a prompt to prepare, not a guarantee.
Sources for the numbers on this page
- Kestrel Instruments — How to prevent plastic shrinkage cracks (Uno evaporation equations and risk bands) · checked October 1, 2026
- National Ready Mixed Concrete Association — CIP 5 — Plastic shrinkage cracking (precautions) · checked October 1, 2026
Corrections and updates are logged on the sources page.